Switching units, in particular circuit breakers, are used inter alia for switching off the power supply in a reliable and safe manner in the event of a short circuit and as a consequence said switching units protect consumers and installations. Moreover, electric or mechanical switching units are suitable for manually switching consumers depending upon the operation involved and also for safely and reliably separating an installation from the power supply network when performing maintenance work or making changes to the installation. Electric switching units in particular circuit breakers are frequently operated in an electromagnetic manner.
In other words, switching units of this type are technically complex electric switching devices having integrated protection for motors, lines, transformers and generators. They are used in applications where switching frequency is at a minimum. In addition to providing short circuit protection, switching units of this type are also suitable for providing overload protection.
In the event of a short circuit, an electric switching unit switches an electric installation off in a safe and reliable manner. Consequently, said electric switching units provide a cut-off protection in the event of an overload. The temperature in each conductor through which current flows increases at different rates. The rate of the temperature increase depends upon the ratio of the current magnitude with respect to the conductor cross section, the so-called current density. The current density must not be too high as otherwise owing to the excessively high temperature the conductor insulation can melt and a fire can possibly result. In order to protect electric installations against the damaging effects, switching units are used as overload protection devices.
Circuit breakers comprise two actuating mechanisms that function separately from one another in order to provide the overload protection and short circuit protection. The two actuating mechanisms are connected in series. One electromagnetic actuating mechanism that functions in an almost non-delayed manner with respect to time provides the short circuit protection. In the event of a short circuit, the electromagnetic actuating mechanism releases without delay a switching lock of the circuit breaker. A switching armature separates the switching piece before the short circuit current can achieve its maximum value.
Known switching units comprise a contact slide unit having a contact slide and a movable switching piece. The movable switching piece comprises further electrical contacts. Moreover, switching units of this type comprise fixed contacts to a current line. In a switched-in state, the electrical contacts of the movable switching piece contact the fixed contacts of the switching unit. In the event of a short circuit, the electrical contacts of the movable switching piece are released from the fixed contacts so that the current flow is interrupted. The movable switching piece is released from the fixed contacts.
Known contact slides of switching units frequently comprise two guide systems, an inner guide system and an outer guide system. The outer guide system is then used if the switching procedure, in other words the connecting procedure or the disconnecting procedure is performed by way of a breaker latching mechanism of the switching unit. The inner guide system is used in the event of a short circuit if the switching procedure is performed by way of a switching armature, frequently by way of a push rod of the switching unit. In other words, during a disconnecting procedure as a result of a short circuit, the movable switching piece moves rapidly along the inner guide system ahead of the contact slide, said movable switching piece impacts against the contact surfaces in the so-called lower part of the switching unit and rapidly returns along the inner guide system. Said movable switching piece moves rapidly against the switching armature or rather the push rod of the switching unit.